Frequency synthesisers have been developed in recent years which permit direct modulation of a carrier signal generated by a Phase Locked Loop (PLL) circuit by rapidly varying the instantaneous value of a variable divider forming part of the PLL circuit; the variable divider is controlled by a digital signal output from a sigma-delta type multi-accumulator digital circuit which acts to shape the noise generated by such a system so that it mostly occurs at higher frequencies where it can be more easily filtered out (by the natural Low Pass Filter behaviour of the closed PLL) before transmission of the signal.
Such frequency synthesisers are able to produce very well controlled modulation with a low enough Signal to Noise Ratio (SNR)—and in particular Signal to Phase-Noise Ratio—at relatively low modulation frequencies to find many practical applications. However, as a result of the type of noise shaping employed by the multi-accumulator digital circuit, it is difficult to maintain such a low SNR where a higher bandwidth is required for the modulation signal. This is because the noise shaping reduces the amount of low frequency noise at the expense of increasing the amount of high frequency noise.
As shown in FIG. 1 of the accompanying drawings, our U.S. Pat. No. 6,211,747 describes a direct modulation multi-accumulator fractional-N frequency synthesiser 1 for generating a modulated RF signal 110 by modulating a carrier signal by a modulation signal 170, 121, the frequency synthesiser comprising a Voltage Controlled Oscillator (‘VCO’) 10 having a tuning port for controlling the frequency of the signal 110 output by the VCO, a variable divider 20 and a sigma-delta multi-accumulator sequence generator 21 for controlling the variable divider 20, a reference frequency generator 50, a phase detector 30 and a low pass filter 40. The output of the reference frequency generator 50 is connected to a negative input 29 of the phase detector 30. The input of the variable divider 20 is connected to receive the output of the VCO 10 and the output of the variable divider 20 is connected to a positive input 28 of the phase detector 30 so that the variable divider 20, the phase detector 30, the low pass filter 40, an adder circuit 41 and the VCO 10 form a Phase Locked Loop (‘PLL’), the directly modulated output signal of which is taken from the output of the VCO; in-band modulation is performed by varying the divide ratio of the variable divider and out-of-band modulation is performed by directly applying the modulating signal to the VCO tuning port through the adder circuit 41.
It will be appreciated that the expression ‘in-band modulation’ refers generally to modulation of the carrier frequency by components of the entire modulation signal whose frequency does not exceed the corner frequency of the low pass filter, while ‘out-of-band modulation’ refers to modulation of the carrier frequency by components of the entire modulation signal whose frequency does exceed the corner frequency of the low pass filter, knowing that the corner frequency is actually a smooth transition. The precise definition of in-band or out-of-band modulation is essentially unimportant in the present context, since the transfer characteristics of the out-of-band modulation are complementary to the transfer characteristics of the in-band modulation.
The synthesiser described in U.S. Pat. No. 6,211,747 offers satisfactory solutions to the problems referred to above. However, we have encountered difficulties, especially with transmitters for signals according to standards such as Enhanced Data for GSM Evolution (EDGE) and Wide-band Code Division Multiple Access (‘WCDMA’), for example, where large bandwidth phase modulation is applied by a PLL and amplitude modulation is applied by a power amplifier. In particular, the VCO may suffer from pull-in effects due to Voltage Standing Wave Ratio (‘VSWR’) changes on the leads to the VCO due to the amplitude modulation, for example. Also, the system may be sensitive to matching of the gain of analogue control of the VCO, due to its variation with temperature and frequency, for example.
There is a need for a frequency synthesiser comprising a dual port modulator for use in a wide-band radio transmitter that provides a cost-effective solution to such difficulties with a minimum power consumption.